1. Field of the Invention
The present invention relates to an exposure determining method. In particular, the present invention relates to a method of determining a photographic printing exposure that is capable of automatically determining, on the basis of the printing exposure condition for a reference film type, the correct printing exposure condition for those film types whose characteristics are different from those of the reference film type; and also relates to an exposure determining method applicable to image copying equipment to determine the exposure effected by the image copying equipment such as an automatic photographic printer (automatic printer) for printing images on color photographs, such as color films, onto color paper.
2. Description of the Related Art
An image formed on a color negative allows, on the whole, the transmission therethrough of light of three colors, i.e., blue (B) light, green (G) light, and red (R) light. It is known from experience that, as a rule, the transmittance of these three color-components are substantially equal to one another, or to one fixed ratio. From this fact, an automatic printer determines a printing light quantity (exposure) on the basis of the following formula: EQU Log Fj=Kj+Dj . . . (1)
where log F represents the logarithm of the printing light quantity, K represents a constant, D represents the large area transmission density (LATD) of the negative which is measured by a photometric system, and j stands for any of B light, G light, and R light.
However, when the printing light quantity is controlled by an automatic printer on the basis of the above stated formula (1), the following problem arises. If a negative used in the photography of a gray object is underexposed, a print produced from this negative has an overall high density as compared with a print produced from a normally exposed negative. On the other hand, if the negative is overexposed, a print produced from this negative has a low density. In order to cope with this problem, a slope control circuit is provided to correct the term Dj in the formula (1) before the exposure is finally determined. However, even with an automatic printer provided with this slope control circuit, defective prints having an incorrect color balance may be produced when the negative used is, for instance, a negative whose properties have changed due to the passage of a long period, a negative subjected to photography using a light source much different from daylight, such as a fluorescent lamp or a tungsten lamp (i.e., a heterogeneous light-source negative), or a negative suffering from color failure. In order to cope with this problem, the term Dj of the formula (1) is corrected during the determination of the exposure. This procedure is called color correction, and there are three manners of color correction, i.e., normal correction, high correction having a higher degree of correction than the normal correction, and lowered correction having a lower degree than the same.
In recent years, many types of high-sensitivity films have been developed, and the number of film types available has increased to several tens. However, the printing exposure conditions required by a plurality of film types are not always the same. In addition, although condition setting films are used to set the condition of the automatic printer with respect to each of various different film types, they cover only a very limited number of film types, which may serve as reference film types. Normally, a condition setting film is a negative having a first portion corresponding to a negative on which an image of a gray object is formed by photographing, and a second portion disposed around the first portion and corresponding to a negative on which an image of an object of a yellow green color close to gray is formed by photographing. Three kinds of condition setting films, i.e., a correct exposure negative, an underexposed negative, and an overexposed negative are available with respect to one reference film type. However, with respect to those film types which have no corresponding condition setting films available, the setting of the respective printing exposure conditions is very difficult and requires a long time with an experienced operator. Further, in order to maintain high print quality, it is essential to suitably manage the printing exposure conditions with respect to each of various different film types. However, this management is difficult when there are many film types. To cope with this problem, certain techniques have been proposed to automatically determine, on the basis of a single reference condition for printing exposure, the correct exposure to be used during printing with respect to each of various different film types.
With respect to films whose properties have changed due to the passage of time or films having various different characteristics (i.e., films having characteristic curves of different configurations), it is known that, if these films are subjected to printing while the exposure is controlled using high correction, correct prints can be produced relatively easily. The resultant print quality, however, is not sufficiently high.
A technique known from Japanese Patent Laid-Open Nos. 1-94927 (1976), 52-20024 (1977), 59-220761 (1984), 61-198144 (1986), etc. is to divide film images into a plurality of portions, measure light with respect to each of the portions, analyze items of data (photometric data) obtained by the mesurement of light, and correct a reference printing exposure condition using selected items of the photometric data so as to determine the exposure condition for use in printing of a film image which is to be printed. According to this known art, if the spectral sensitivity distribution in the light-measuring portion of the photometric system ofthe automatic printer accords with that of a printing photosensitive material with a very high degree of precision, it is possible to subject a plurality of types of films having different characteristics, to printing on the basis of the printing exposure condition corresponding to a reference film type.
If such is the case, i.e., if the spectral sensitivity distribution of the photometric system accords with that of the exposure portion, it is possible to effect correct printing exposure with respect to each of several film types on the basis of the printing exposure condition for the reference film type, only within the linear portions of the characteristic curves of the relevant film with respect to R, G and B lights. For this reason, the above described known art fails to produce good printed images within the exposure region that corresponds to the non-linear portions of the characteristic curves. Concerning the non-linear portions of the characteristic curves, Japanese Patent Laid-Open No. 49-29641 (1974) proposes an electrical circuit for correcting a non-linear portion at the upper or lower end of a characteristic curve into straight lines approximating the curve. With this art, however, since the non-linear portion of a characteristic curve is corrected into straight lines, it still fails to produce good printed images within the exposure region corresponding to the non linear portions of the characteristic curves.
The reasons why it is impossible to produce good printed images within the exposure region corresponding to the non-linear portions of the characteristic curves of a film will be described in detail with reference to FIG. 14. It is assumed that a reference film type has characteristic curves with respect to three colors, which are substantially identical with those characteristic curves with respect to G and R shown in FIG. 14, while a film to be subjected to printing has characteristic curves which are substantially identical with the characteristic curves with respect to R, G and B shown in FIG. 14. In this case, since the characteristic curves of the reference film type are such that the gradient of the curve portions in the overexposure range is smaller than that of the linear portions, a relatively small slope control value is set within the overexposure range. However, if the relevant film is subjected to printing using the set slope control value, the set slope control value is too small for the characteristic curve of the film with respect to B, thereby resulting in a shortage of the exposure of the blue sensitive layer of printing paper. As a result, the yellow pigment fails to emit color appropriately, and the resultant print tends to be bluish on the whole. In this way, in the above described case, it is impossible to produce good printed imaqes.
A problem similar to that described above concerning the overexposure range arises also with respect to the underexposure range. Specifically, in the above described case, since those portions of the characteristics curves of the relevant film corresponding to the underexposure range deviate from those of the reference film type, this also leads to the problem that good printed images cannot be produced.
As described above, when the characteristics with respect to R, G and B of a film to be subjected to printing are such that the balance in density between the three colors R, G and B differs from the balance in density between the three colors possessed by the reference film type used, there is the risk that, on the resultant print, the complementary color of the color causing the inter-film difference in the three color density balance may be emitted only to an excessive or insufficient extent. Thus, the difference in the three color density balance of a film to be subjected to printing from that of the reference film type used makes it impossible to produce good printed images.
Other differences in characteristics between a film to be subjected to printing and the reference film type used makes it difficult to produce good printed images. Certain films, such as a film whose characteristic curves have a steeper gradient than that of the reference film type used, or a film whose mask density (i.e., base density) is higher than that of the reference film type used, possess higher densities than the reference film type even in the overexposure range. Accordingly, the corresponding exposure time is long, and this makes the density achievable on print paper, highly vulnerable to influence by the failure of the reciprocity law. In the case of the reference film type, the determination of the exposure condition normally employs a slope controlling function in the overexposure range, thereby compensating for the possible influence by the failure of the reciprocity law. However, when the film to be subjected to printing possesses a higher density than the reference film type in the overexposure range, it is impossible for a slope controlling function to appropriately compensate for influence by the failure of the reciprocity law on print paper. In this way, when a film whose characteristic with respect to a certain color corresponds to a higher density than the reference film type in the overexposure range, a print produced from the film suffers from an insufficient print density with respect to that color, and fails to provide a good printed image.
Various difficulties are met with in determining the printing exposure condition for negatives of certain kinds. As described above, defective prints having an incorrect color balance may be produced by an automatic printer provided with the above-described slope control circuit, when the negative used is, for instance, a negative whose properties have changed due to the passage of a long period of time, a negative subjected to photography using a light source much different from daylight, such as a fluorescent lamp or a tungsten lamp (i.e., a heterogeneous light-source negative), or a negative suffering from color failure. In addition, when it is necessary to process heterogeneous films, i.e., films manufactured by different manufacturers, or films having differing sensitivities, because the three photo-sensitive layers of such heterogeneous films have different sensitivities, densities, etc., it is impossible to produce good prints under the same printing condition. In practice, therefore, various printing exposure conditions are determined with respect to various different film types by trial and error. They are then stored in a memory, and the printing condition corresponding to the film type to be subjected to printing is selected to produce prints. During the determination of the exposure, the term Dj of the formula (1) is corrected, thereby effecting color correction. If heterogeneous films are to be processed, another procedure takes place where the slope control circuit value is varied.
Methods of a certain type have hitherto been known as an improvement of the above described techniques for determining the exposure. In the methods photometric data is obtained by measuring light with respect to a plurality of portions into which original images on color photographs are divided, and the resultant photometric data is evaluated. These methods fall into two categories in accordance with the manner in which the photometric data is evaluated. Methods in the first category compare items of photometric data with a reference value, and determine the printing exposure solely on the basis of those items of photometric data selected on the basis of the result of this comparison. Methods in the second category determine the printing exposure by taking all the photometric data into consideration.
Examples of methods in the first category are disclosed in Japanese Patent Publication Nos. 56-15492 (1981) and 59-29847 (1984), and Japanese Patent Laid-Open Nos. 52156624 (1977), 53-1230 (1978), 58-118636 (1983), 59-220760 (1984), and 59-220761 (1984). In the art known from these proposals, those items of photometric data which have not been selected are not included in the determination of the exposure.
Consequently, the exposure is determined only on the basis of a small number of items of photometric data when only a small number of items of photometric data are selected from among all the items of data obtained concerning the images to be processed because, for instance, the images to be processed have colors greatly deviating from certain standards. In such cases, the precision with which the exposure is determined cannot always be high, thereby leading to a risk of the resultant print suffering from problems with colors. With a method in the first category, the number of items of photometric data selected can be small due to deviation in colors when the film used has been subjected to photography using a heterogeneous light source such as a fluorescent lamp or a tungsten lamp, or when it has been subjected to photography in the sunlight but at a low color temperature, e.g., in the evening sunlight or winter sunlight. In such cases, the resultant print tends to have colors with emphasis on the color of the light source used during photography. Thus, a method in the first category inevitably leads to a degradation of print quality when the negative used is a heterogeneous light-source negative or the like. Further, with the method, when the images to be processed have greatly deviated colors, there is the risk that all the items of photometric data may be excluded by the selection procedure. If such is the case, the mean values of the averages of the B, G and R densities of the images which are calculated with respect to three colors are often used. With this method, however, since information on the actual colors of the photographic image is lost, it is impossible to produce prints with good colors.
Still further, when the images to be processed have greatly deviated colors, if all the items of photometric data are excluded by the selection procedure, and the three-color mean values of the averages of the B, G and R densities of the images are used, since these values do not represent the characteristics of the photographic images, only a low level of correction is possible when the images to be processed are formed on films of various different types.
An example of a method in the second category is disclosed in Japanese Patent Laid-Open No. 61-198144 (1986). In this method, items of the photometric data are sorted out by determining whether each item of data belongs to a high-saturation region or a low-saturation region, and the exposure is calculated on the basis of the above-stated formula (1) in which is employed the weighted mean Dj of the mean values MDH and MDL of photometric data belonging to the corresponding regions. The weighted mean Dj is expressed by the following formula: EQU Dj=Ka MDH+Kb.multidot.MDL EQU Ka+Kb=1 . . . (2)
In this case, since it is necessary to suppress the influence of the mean value MDH concerning the high-saturation region, the value of the constant Ka is set within the range from 0.0 to 0.4. However, when the images to be processed have greatly deviated colors, the mean value MDH concerning the high saturation region greatly influences the determination of the exposure, thereby making it impossible to appropriately compensate for the color failure. On the other hand, in the case of processing images formed using heterogeneous light sources, compensation can be effected by changing the values of the coefficients Ka and Kb. However, a determination has to be made as to whether a heterogeneous light source was used or not, and if a wrong determination is made, the resultant printed images will deteriorate to a great extent. Japanese Patent Laid-Open No. 61-223731 (1986) proposes to determine the PG,15 exposure by employing as the mean value MDH in the abovestated formula (2) a value Dw indicative of an achromatic color. However, this method also encounters a similar problem. Since the value Dw is obtained on the basis of the mean value MDH concerning the high-saturation region, the correction provided proves to be inappropriate with respect to films of various different film types or heterogeneous light-source negatives and, hence, inappropriate for the characteristics of the color images on such films.